Anodes or electrodes for electrolytic tantalum capacitors are manufactured by compacting or compressing a precise quantity (weight) of capacitor-grade tantalum powder, which has been placed inside a die cavity to provide a coherent shape, sintering the compact, and subsequently anodizing the sintered compact to form a continuous dielectric oxide film. Usually, a tantalum wire lead is inserted into the die cavity as the compressing punches press the powder contained in the die cavity. The anodes are required to have a precise weight because the powder is rated in terms of its specific capacity in microfarad-volts per gram--i.e., a given amount of a specific powder processed by a specified fixed pressing, sintering and anodizing procedure will produce a precise surface area and capacitance.
The powder must have sufficient flowability to enter and uniformly fill the die cavity in a short time to the required weight of tantalum powder. Flowability is measured as the rate, in grams per second, at which a powder will flow through an orifice in a standardized test. If the powder has poor flowability, it passes through the orifice at a low rate, or it may not flow through the orifice at all unless external vibratory motion is applied. Low flowability powders may not fill the die cavity homogeneously because, as they flow into the cavity volume, they may not spread evenly across the cross section of the cavity. This unevenness in filling of the cavity, along with the low rate at which the cavity fills, results in variations in anode weight and inhomogeneity in density of the pressed pellet, which adversely affect the effective surface area and produce substantial capacitance variation among the resulting anodes. Anodes for capacitors used in miniaturized printed circuits contain very small amounts of powders that have a very large specific capacity. The powder pressing dies for such parts have small dimensions. Thus, there is a growing need for powders with good flowability.
As shown in an example of this disclosure, state-of-the-art powders exhibit poor flowability for many current applications, and require that a binder be added to the powder to achieve adequate flowability. In this process, tantalum powder is admixed with a binder substance that makes the powder particles come close together by virtue of the adhesion properties of the binder. Binders commonly employed for tantalum powder have included: camphor, stearic and other soapy fatty acids, Carbowax (Union Carbide), Glyptal (General Electric), polyvinyl alcohols, napthaline, vegetable wax, and microwaxes (purified paraffins). The binder is dissolved and dispersed in a solvent. Solvents commonly used have been: acetone; methyl isobutyl ketone; trichloromethane; fluorinated hydrocarbons (freon) (DuPont); alcohols; and chlorinated hydrocarbons (carbon tetrachloride). The binder and solvent solution is admixed with the tantalum powder and further processed to produce solid granules, typically about -30 mesh to +200 mesh sieve size. Such granules are free flowing at room temperature and have the general appearance of spheroids. These granules are fed to a press and compacted in dies to produce the anode compacts.
The binder that has been added to enhance flowability must later be removed by means of temperature and reduced pressure. The anodes typically are heated in a vacuum chamber to sublime and vaporize the binder substances. There are a number of problems attendant to the use of binders:
A) Too-fast vaporization or sublimation causes rapid gas evolution that can fracture or bloat the anode compacts to render their dimensions useless.
B) If the binder decomposes due to temperature or pressure, the resulting carbon, oxygen and hydrogen residuals can contaminate the tantalum and adversely affect dielectric quality.
C) If the binder is not completely removed in the debinderizing process, residual amounts can cause excessive outgassing during sintering, potentially contaminating and embrittling the furnace parts or contaminating the diffusion pump oils.
D) Compounds used as binders and binder solvents present potential health and safety risks. They require large equipment and manpower expenditures to minimize health risks, and to prevent fires, explosions and damage to the products and equipment.
E) The compounds used as binders are very sensitive to small changes in temperature. This is because their surface tensions change rapidly with small environmental temperature changes, causing additional difficulties to the manufacturer. Materials containing binders must be maintained at a low and controlled temperatures.
F) The mechanical compact-forming press develops heat due to frictional forces on its moving parts. This is particularly a problem with the powder-filling shoe that continuously slides over the face of the die. This heat transports to the binderized powder within the shoe, changing the binder surface tension and converting the binder to a high viscosity fluid or "grease". The granules become plastic, stick together and lose their flow characteristics. When this occurs, the "grease" coats critical areas of the equipment so it does not function at prescribed rates and must be stopped for cleaning. In addition, when the binderized powder changes from its dry solid condition to its "greased" state, it cannot flow homogeneously into the die cavity, resulting in large variations in weight, surface area and capacitance among the anodes.
Some efforts have been made to improve the flowability of tantalum powder by adding extraneous inorganic substances instead of binders. According to U.S. Pat. Re. No. 32,260, additions of calcium orthophosphate to tantalum powder were found to improve flow in a modified orifice flow test.
G) When state-of-the-art tantalum powders are compacted in mechanical presses to produce pellets for preparation of anodes for electrolytic capacitors, a number of difficulties are experienced in the pressing operation, in the subsequent sintering operation, and in quality deficiencies of the anode and capacitor produced therefrom, including:
1) Flow of the powder is poor or inadequate, especially when efforts are made to press the powder to pellets in high speed, mechanical presses.
2) Weight control and pellet uniformity can be poor.
3) The powder tends to smear on the pellet surface (i.e., die wall to pellet interface).
4) Shrinkage during sintering can be excessive, causing an unacceptable loss in capacitance and variability.
5) Because of the smearing problem (item 3, above), penetration of manganese dioxide into the anode surface, which is required in a later solid tantalum capacitor preparation operation, is inhibited due to loss of surface porosity in the anode. This same problem also is believed to adversely affect electrical parameters in the capacitor, including capacitance, tangent theta (.delta.), impedance and ESR. The capacitance loss can be especially noted in the wet-to-solid capacitance loss.